Lithium–Iron Fluoride Battery with In Situ Surface Protection

Abstract

Lithium-metal fl uoride (MF) batteries offer the highest theoretical energy density, exceeding that of the sulfur-lithium cells. However, conversion-type MF cathodes suffer from high resistance, small capacity utilization at room temperature, irreversible structural changes, and rapid capacity fading with cycling. In this study, the successful application of the approach to overcome such limitations and dramatically enhance electrochemical performance of Li-MF cells is reported. By using iron fl uoride (FeF 2 ) as an example, Li-MF cells capable of achieving near-theoretical capacity utilization are shown when MF is infi ltrated into the carbon mesopores. Most importantly, the ability of electrolytes based on the lithium bis(fl uorosulfonyl)imide (LiFSI) salt is presented to successfully prevent the cathode dissolution and leaching via in situ formation of a Li ion permeable protective surface layer. This layer forms as a result of electrolyte reduction/oxidation reactions during the fi rst cycle of the conversion reaction, thus minimizing the capacity losses during cycling. Postmortem analysis shows the absence of Li dendrites, which is important for safer use of Li metal anodes. As a result, Li-FeF 2 cells demonstrate over 1000 stable cycles. Quantum chemistry calculations and postmortem analysis provide insights into the mechanisms of the passivation layer formation and the performance boost.

In situ surface protection for enhancing stability and performance of conversion-type cathodes

“…[21][22][23]43,45 Similar to Li-chalcogen batteries, recent studies on various MHs also reported serious challenges of metal and halide dissolution during electrochemical conversion reactions. 16,17,[46][47][48][49][50] Nearly all conversion type cathode materials are somewhat soluble in polar organic solvents in at least some potential range. For example, CuF 2 -based cathodes suffer from severe Cu dissolution during charge before the reversible potential for the CuF 2 formation may be reached, which so far prevents having more than one electrochemical cycle of the unprotected CuF 2 .…”

“…Like CuF 2 , both FeF 2 and CoF 2 have similar (although less severe) cation dissolution problem, which have been recently discussed. 17,46 Compared with MFs, the direct dissolutions of metal chloride (MCl) cathodes are even more serious, which hinder their development despite their smaller charge-discharge voltage hysteresis, higher rates, and better energy efficiencies. LiCl (the discharge product of MCl) is highly soluble in most organic solvents.…”

“…As such, if cathode dissolution and unfavorable interactions between active materials and electrolyte are avoided, conversion cathodes should be able to show very long cycle stability in cells. [16][17][18][19][20][21][22][23][24][25][26][46][47][48][49][50] The in situ CEI protection on the surface of the conversion cathodes has a promise to overcome such challenges as shown in Fig. 3(b).…”

“…Identifying various favorable combinations of suitable (i) main (Li) 46,55 or (ii) additive (either Li or non-Li) 39,45 salts, (iii) organic 17,47 and (iv) inorganic 39,56-58 additives and (v) electrolyte solvents 53,59 are promising strategies to induce formations of such favorable CEI layers. Several criteria may be important for identifying suitable electrolyte compositions for the formation of surface-protective CEI layers (Fig.…”

“…Example of using a LiFSI as the main Li salt for the cathode SEI formation and the performance improvement of Li-S battery cells: SEM micrographs of S/C cathode after 150 cycles with (a) 2.4 M LiTFSI in DIOX:DME and (b) 2.3 M LiFSI in DME electrolytes, respectively, showing smooth SEI formation in the later; typical XPS scans of F 1s spectrum on Li anode and S cathode surface respectively after cycling in different concentrations of (c) LiTFSI and (d) LiFSI-based electrolytes, showing preferred formation of LiF in the later; (e) accelerated electrochemical stability tests conducted at 60 °C and comparing performance of LiTFSI and LiFSI electrolytes in either a single DME solvent or a DME:DIOX mixture; (f) oxidation reactions between FSI(-F) radical and DME from QC calculations. 46 average coulombic efficiency (CE) approaching 100.0% and capacity retention of 77% after 1000 cycles (≈10% degradation during cycles 100-1000) [see Fig. 7(e)].…”